Marine

G.I.

Special ability: Deploy. Increases damage to 25 and decreases attack cooldown to 15.

Firstly, it is clear that we can't directly compare the two using these numbers, so we'll amplify the stats of both units to equalize them as best as we can.

Marine

G.I.

Deployed G.I

HP

120

125

125

Damage

18

15

25

Misc

-30HP with stimpack

First we shall compare how much damage is done per frame.

Marine

Stimpack Marine

G.I

Deployed G.I.

1.2

2.4

0.75

1.7

And now, we can see how fast a unit can kill the other

Marine

Stimpack Marine

G.I.

Deployed G.I.

Frames needed to kill

104.2

52.1

160

(120 on stimpack)

70.6

(52.9 on stimpack)

A stimpack lasts for 9 seconds, which is equalavent to 207 frames, and thus, one usage of stimpack is enough for a 1 vs 1 encounter.

With this final table, we can finally answer the question.

A marine vs undeployed G.I. Results in the marine wins easily.

A marine vs deployed G.I. Results in the deployed G.I. wins easily.

A marine stimpacks before battle vs Deployed G.I. At first glance, it looks as if both units will kill each other, but we have to recall that the Stimpack Marine shoots every 7.5 frames, and he will be able to shoot on frame 52.5, killing the G.I before the G.I can shoot on frame 60. And so, the Starcraft Marine will live with a slither of health.

We have to consider the human factor, however, as the difference of 7.5 frames is a very short amount of time. In game, Starcraft Fastest runs at 22 frames per second. That means that you have 0.34 seconds to use the stimpack on the marine to come up on top. The average reaction time of humans is 0.25 seconds, which means it's very possible for the marines to use the stimpack as soon as they fight against the G.I, but in most scenarios, both units will kill each other. However, if the stimpack was used a second late, the deployed G.I. will be able to kill the Marine with a slither of health left.

Conclusion

Marine wins.

In an open field, without any buildings for G.I. to take cover inside, an open fight between an equal amount of marines vs an equal amount of deployed G.I. will result in the victory of the marines. The huge advantage Marines have in shortening their attack cooldown ended up the key for their victory.

That said, the G.I. almost nearly came up on top in this fight, and in an actual game, would most likely come on top against the marines due to human error.